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Full text of "The Cambridge Natural History"

CHEEK TEETH

the biting surface ends. The grinding teeth vary from simple
one - cusped teeth, precisely like canines, to teeth with an
enormous number of separate tubercles. In the former case it is
hard to distinguish between incisors, canines, and cheek teeth
in the lower jaw, where no suture separates the bone. More- 
over it is quite common for the first cheek tooth in the lower
jaw to have the characters of a canine, while the true canine
approximates in its form to the antecedent incisors. This is
so, for instance, with the Lemurs, where the first premolar is
eaniiiiforin, and the canine shares in the curious procumbent
attitude which distinguishes the lower incisors of many of those
animals.
A variable number of the anterior cheek teeth may be little
more than simple conical teeth ; but the rest of the set are
commonly more complicated. "No definite laws can be laid down
as to the complication of the posterior as compared with the
anterior set. Broadly speaking, it is purely herbivorous creatures
in which the least difference can be detected at the two ex-
tremities, and which are at the same time the most elaborately
decorated with tubercles and ridges. The converse is true that
in purely carnivorous animals,, including insect- and fish-eating
forms, there is the greatest difference between the anterior set
of grinding teeth and those which follow. In these two respects
such animals as a Lemur and a Rhinoceros occupy the extremes.
Furthermore, it may be said that omnivorous creatures lie, as
their diet would suggest, in an intermediate position. Generally
speaking, when there is a marked difference between the first
premolar "and molars at the end of the series, there is a gradual
approximation in structure of a progressive kind. The tubercles
become more numerous in successive teeth; but the corollary
which is apparently deducible from this, i.e. that the last molar
is the most elaborate of the series, is by no means always true.
The last cheek tooth indeed is often degenerate. On the other
hand, it is very markedly the largest of the series in such diverse
types as the Elephant, the hog I^iacochoerus, and the Jfodent
Ifydrochoerus. It is a rule that the cheek teeth of the upper jaw
are more complicated than the corresponding teeth of the lower
jaw.
The structure of the cheek teeth is very diverse among the
Mammalia. ~Brcj,dly, two types are to "be recognised. There are